The human spine is a complex mechanical structure composed of alternating bony vertebrae and fibrocartilaginous discs that are connected by strong ligaments and supported by musculature that extends from the skull to the pelvis and provides axial support to the body. The intervertebral discs primarily serve as a mechanical cushion between adjacent vertebral segments of the spinal column and generally comprise three basic components: the nucleus pulposus, the anulus fibrosis, and two vertebral end plates. The end plates are made of thin cartilage overlying a thin layer of hard cortical bone that attaches to the spongy, cancellous bone of the vertebral body. The anulus fibrosis forms the disc's perimeter and is a tough outer ring that binds adjacent vertebrae together. The vertebrae generally comprise a vertebral foramen bounded by the anterior vertebral body and the neural arch, which consists of two pedicles and two laminae that are united posteriorly. The spinous and transverse processes protrude from the neural arch. The superior and inferior articular facets lie at the root of the transverse process. The term “functional spinal unit” (“FSU”) refers to the entire motion segment: the anterior disc and the posterior facet joints, along with the supporting ligaments and connective tissues.
The spine as a whole is a highly flexible structure capable of a high degree of curvature and twist in nearly every direction. However, genetic or developmental irregularities, trauma, chronic stress, and degenerative wear can result in spinal pathologies for which surgical intervention may be necessary.
It is common practice to remove a spinal disc in cases of spinal disc deterioration, disease or spinal injury. The discs sometimes become diseased or damaged such that the intervertebral separation is reduced. Such events cause the height of the disc nucleus to decrease, which in turn causes the anulus to buckle in areas where the laminated plies are loosely bonded. As the overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur. Such disruption to the natural intervertebral separation produces pain, which can be alleviated by removal of the disc and maintenance of the natural separation distance. In cases of chronic back pain resulting from a degenerated or herniated disc, removal of the disc becomes medically necessary.
In some cases, the damaged disc may be replaced with a disc prosthesis intended to duplicate the function of the natural spinal disc. U.S. Pat. No. 4,863,477 discloses a resilient spinal disc prosthesis intended to replace the resiliency of a natural human spinal disc. U.S. Pat. No. 5,192,326 teaches a prosthetic nucleus for replacing just the nucleus portion of a human spinal disc.
In other cases it is desired to fuse the adjacent vertebrae together after removal of the disc, sometimes referred to as “intervertebral fusion” or “interbody fusion.”
Many techniques and instruments have been devised to perform intervertebral fusion. There is common agreement that the strongest intervertebral fusion is the interbody (between the lumbar bodies) fusion, which may be augmented by a posterior or facet fusion. In cases of intervertebral fusion, either structural bone or an interbody fusion cage filled with morselized bone is placed centrally within the space where the spinal disc once resided. Multiple cages or bony grafts may be used within that space.
Such practices are characterized by certain disadvantages, most important of which is the actual morbidity of the procedure itself. Placement of rigid cages or structural grafts in the interbody space either requires an anterior surgical approach, which carries certain unavoidable risks to the viscous structures overlying the spine (intestines, major blood vessels, and the ureter), or they may be accomplished from a posterior surgical approach, thereby requiring significant traction on the overlying nerve roots. The interval between the exiting and traversing nerve roots is limited to a few millimeters and does not allow for safe passage of large intervertebral devices, as may be accomplished from the anterior approach. Alternatively, the anterior approach does not allow for inspection of the nerve roots, is not suitable alone for cases in which the posterior elements are not competent, and most importantly, the anterior approach is associated with very high morbidity and risk where there has been previous anterior surgery.
Another significant drawback to fusion surgery in general is that adjacent vertebral segments show accelerated deterioration after a successful fusion has been performed at any level. The spine is by definition stiffer after the fusion procedure, and the natural body mechanics place increased stress on levels proximal to the fused segment. Other drawbacks include the possibility of “flat back syndrome” in which there is a disruption in the natural curvature of the spine. The vertebrae in the lower lumbar region of the spine reside in an arch referred as having a sagittal alignment. The sagittal alignment is compromised when adjacent vertebral bodies that were once angled toward each other on their posterior side become fused in a different, less angled orientation relative to one another. Finally, there is always the risk that the fusion attempt may fail, leading to pseudoarthrosis, an often painful condition that may lead to device failure and further surgery.
Conventional interbody fusion cages generally comprise a tubular metal body having an external surface threading. They are inserted transverse to the axis of the spine, into preformed cylindrical holes at the junction of adjacent vertebral bodies. Two cages are generally inserted side by side with the external threading tapping into the lower surface of the vertebral bone above, and the upper surface of the vertebral bone below. The cages include holes through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior of the cage to incite or accelerate the growth of the bone into the cage. End caps are often utilized to hold the bone graft material within the cage.
These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. As previously discussed, however, cages that would be placed from the safer posterior route would be limited in size by the interval between the nerve roots. It would therefore, be a considerable advance in the art to provide a fusion implant assembly which could be expanded from within the intervertebral space, thereby minimizing potential trauma to the nerve roots and yet still providing the ability to restore disc space height.
Ultimately though, it is important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient's spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. Thus, it would be an even greater advance in the art to provide an implant assembly that does not promote fusion, but instead closely mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.